During the manufacture of integrated devices, a number of technological steps are present (such as, reactive ion-etching (RIE) or plasma deposition and/or etching) which induce the charging of certain layers of the wafer being processed, and specifically of the most exposed layer. When the most exposed layer is a conductor layer (such as defining polycrystalline silicon or metal lines), the voltage is transferred along the entire conductive line, and the system tends to discharge through the weakest point, usually represented by the gate oxide region. This situation is, however, undesirable in that it jeopardizes the reliability of the final device.
Proposed solutions to the foregoing include:
reducing the "antenna ratio", defined by the ratio between the area of the conductive path and the area of the gate oxide region. In this way, the total capacity of the conductor layer that discharges through the gate oxide region is decreased, and thus the amount of charge present, given the same voltage, decreases; PA1 inserting along the interconnections suitable N.sup.+ /substrate diodes or P.sup.+ /N well diodes which limit the maximum voltage that may be reached by the conductor layer and, if forward biased, prevent the conductor layer from being charged with negative voltage.
The problem existing in the manufacture of protection diodes normally inserted in integrated devices consists in the fact that the range of allowed voltages (for which there is no protection) is too wide as compared with current requirements, and they are limited by the diode breakdown voltage, typically higher than 10 V. On the other hand, the presence of voltages higher than 10 V on gate oxide regions having a thickness of 12 nm corresponds to the application of electric fields of over 8 MV/cm. In the case of gate oxide layers having a thickness of 7 nm, there are electric fields even greater than 14 MV/cm. These voltages are much higher than normal operating conditions and are even higher than the values at which the Fowler-Nordheim tunnel effect conduction mechanisms start, which may lead to degradation of the oxide regions. In practice, with the thicknesses currently envisaged, traditional diodes are not able to intervene before voltages dangerous for the oxide layers are set up, and hence do not provide an effective protection against damages to the gate oxide layers.